The purpose of this study was to evaluate the accuracy of left ventricular ejection fractions (LVEFs) obtained using gated planar imaging of dynamic cardiac phantoms and measured with a hybrid boundary detection technique. Water mixed with Tc-99m pertechnetate was used to fill the ventricles, and from 19 ml to 52 ml was added to the LV chambers at end diastole so that LVEFs of 24% to 56% were produced. Three different low energy collimators (high resolution, ultra-high resolution, and super high resolution parallel beam collimators) were evaluated. A Metz filter was used for spatial smoothing and a hybrid boundary detection algorithm was used for generating regions of interest (ROIs) in the twenty frames of images per R-R interval accumulated from 410 to 460 beats. The boundaries of the LV chambers were determined by combined first- and second-difference operators weighted by a hybrid enhancement weight α(0<α<1). The counts in the ROIs were used to calculate LVEFs. In general, the higher the value of a, the greater the deviations from actual EFs. The second-difference (α=0) method resulted in more accurate values than the first-difference (α=1) method. No large differences in the EF measurements were observed between the collimators. In the presence of background activity, the nearly optimal weight for three different collimators was α=0.5